Guide wires are used to guide catheters for use in the treatment of body regions that are difficult to operate on surgically, such as PTCA (Percutaneous Transluminal Coronary Angioplasty), the treatment of body regions through minimally invasive surgery, and the inspection of blood vessels by cardiac angiography. For example, for performing PTCA using a guide wire, the distal end of the guide wire is positioned to project from the distal end of a balloon catheter, and the guide wire together with the balloon catheter is inserted into the blood vessel up to a position near the constricted area of the blood vessel in question for guiding the distal end of the balloon catheter to the position near the constricted area.
Blood vessels oftentimes possess a complexly curved shape. A guide wire used to insert a balloon catheter into a blood vessel is required to be pliable and recoverable when it is bent, to possess pushability and torque transmittability characteristics (collectively referred to as “operability”) for transmitting an action or operation (force) on the proximal end of the guide wire to the distal end of the guide wire, and to exhibit kink resistance qualities (bend resistance), etc. Particularly for imparting pliability characteristics to the guide wire, it has been proposed to use a metal coil which is pliable when bent, disposed around a thin distal end core of the guide wire and a superelastic wire of Ni—Ti or the like used as the core of the guide wire to make the guide wire pliable and recoverable.
Known guide wires have a core made essentially of one type of material. In order to increase the operability of the guide wire, the core is made of a material having a relatively high modulus of elasticity. As a result, the distal end of the guide wire tends to be less pliable. If the core of the guide wire is made of a material having a relatively low modulus of elasticity in order to make the distal end of the guide wire sufficiently pliable, the operability of the guide wire at the proximal end is lost. A need exists for a guide wire which preferably meets the requirements for both pliability and operability.
In an attempt to address such need, there has been proposed in U.S. Pat. No. 6,001,068 a guide wire including a flexible first wire disposed on a distal end side and a highly rigid second wire disposed on a proximal end side, the first and second wires being joined to each other by brazing. This guide wire possesses increased pliability at the distal end and increased rigidity at the proximal end for better operability.
However, if the first wire and the second wire are covered with an oxide film, the brazing material fails to sufficiently wet the oxide film. As a consequence, it is hard to securely braze the first wire and the second wire to each other. Since Cr and Ti form a chemically stable oxide film, they are liable to produce the above tendency. Also, it is particularly difficult to firmly braze wires that are made of stainless steel or Ni—Ti-based alloy.
Attempts have been made to remove the oxide film by using a flux for reducing the oxide film. However, as the flux fails to sufficiently remove the oxide film, the bonding strength of the brazed joint formed using the flux is not large enough.